The present invention relates to a method for manufacturing a thin-film electroluminescent element used in indication devices.
A thin-film electroluminescent element provides electroluminescence (hereinafter abbreviated as EL) as a result of applying an electric field. Such elements are known to be useful as a display panel for thin indication devices because of the capability of the element for high brightness luminescence, high resolution, and variable displays on a large screen. The construction of the conventional thin-film EL element is shown in a cross section view in FIG. 2 and in a plan view in FIG. 3. Transparent indium-tin oxide ("ITO") electrodes 2 with a thickness of 2000 angstroms are formed in parallel strips on a glass substrate 1. A first insulation film 3 with a thickness of 3000 angstroms is laminated upon the substrate, using a material such as Si.sub.3 N.sub.4 and a technique such as a sputtering process. A light-emitting layer 4 with a thickness of 5000 angstroms is then applied, using a material such as ZnS:Mn and a technique such as an electron beam deposition process. A second insulation film 5 is then laminated upon the light-emitting layer, again with a thickness of 3000 angstroms, using a material such as Si.sub.3 N.sub.4 and a technique such as a sputtering process. An aluminum film is then applied with a thickness of 5000 angstroms using a sputtering process, and patterned to form rear electrodes 6 in parallel strips oriented perpendicular to the transparent electrodes 2.
Picture elements exist at the intersections of the transparent electrodes 2 and the rear electrodes 6. When an AC electrical field is applied across the transparent electrodes 2 and the rear electrodes 6, the picture elements emit light.
In order for such an EL element as described above to be used in practical applications, it is necessary to have electrode terminals 21 disposed on the ends of the transparent electrodes 2 and the rear electrodes 6 in order to make an electrical connection with the driving integrated circuits. Thus, it is important that the terminals 21 on the transparent electrodes 2 not be covered by the first insulation film 3, the light emitting layer 4, or the second insulation film 5. Further complicating the manufacture of the element is the fact that the light emitting layer 4 is generally made from materials such as ZnS, CdS, or SrS combined with a transition metal such as Mn or rare earth elements such as Tb, Sm, Ce and Eu. These light emitting materials dissolve easily in acidic or alkaline aqueous solutions, for example the mixture of phosphoric acid and nitric acid frequently is used to pattern the rear electrodes 6. If the light-emitting layer 4 dissolves as a result of making contact with an aluminum etching solution, film peel-off occurs on the second insulation film 5 and on the rear electrodes 6. Thus, both the manufacturing yield of the EL elements and the reliability of the elements decrease. For this reason the light-emitting layer 4 must be covered with the second insulation film 5 as shown in FIG. 2.
In a known method for obtaining EL elements, in order to obtain the patterns for each layer as described above, the ITO films disposed on the glass substrate 1 are patterned in parallel stripes to form transparent electrodes 2, whereupon the transparent electrode terminals 21 are covered with metallic masks to shield the terminals 21 while the first insulation film 3 is formed by a sputtering process. The metallic masks are then replaced with metallic masks that create an opening of smaller size than the opening created by the initial masks, whereupon the light emitting layer 4 is deposited by electron beam deposition. Next, the metallic masks are again replaced with masks that create an opening as large as the opening created by the initial masks used in forming the first insulation film 3, whereupon the second insulation film 5 is disposed using the sputtering process. The metallic masks are then removed, whereupon a layer of aluminum is disposed using a sputtering process, and is then patterned using an etching process to form the rear electrodes 6.
This method for manufacturing such conventional EL elements requires that the masks be replaced upon the formation of the first insulation film 3, the light emitting layer 4, and the second insulation film 5. Additionally, the substrate must be removed from a vacuum tank used by the sputtering equipment or the deposition equipment after the formation of each layer. Consequently, conventional manufacturing methods suffer from low productivity. Additionally, the high frequency of mask replacement creates the risk of contamination of the substrate surface, causing a reduction in the yield. All of these problems lead to an increased cost of manufacture.
It is an object of the present invention to solve these problems and to provide a method for manufacturing an EL element with high productivity and high yield while keeping the transparent electrode terminal exposed and the light emitting layer covered with the second insulation film.